With semiconductor manufacturing facilities and chemical products manufacturing facilities, it is required that the flow rate and pressure of raw gases to be supplied are controlled with the high degree of accuracy. To meet the needs, various types of pressure control devices, flow rate control devices and pressure sensors to be employed for these devices have been developed.
FIG. 13 and FIG. 14 illustrate one example of the conventional flow rate control device. With FIG. 13 (the U.S. Pat. No. 5,146,941), gas pressure P1 on the upstream side from the orifice F and differential pressure δP between the inlet side of the orifice F and the throat part are inputted to a computing means C, and the gas flow rate on the downstream side from the orifice is controlled to be the set flow rate by the open/close control of a control valve V through a valve controller VC based on the flow rate Wg computed with the computing means and the set flow rate Wr. This is what is known as a so-called differential pressure type flow rate control device
Similarly, FIG. 14 (TOKU-KAI-HEI No. 8-338546) illustrates another example of the conventional pressure type flow rate control device. This is what is publicly known as a pressure type flow rate control device to be used under critical conditions (P2/P1≦approx. 0.5), wherein the gas flow rate on the downstream side from the orifice under critical conditions is computed as Qc=KP1 (where P is the pressure on the upstream side from the orifice) by the computing means C, and the control valve V is controlled by the open/close control to make smaller the difference between the set flow rate Qs and the afore-mentioned computed flow rate Qc, thus the gas flow rate on the downstream side from the orifice F is controlled to be the set value.
With the flow rate control device and the like as described above, it is needed that the gas pressure P1 and the like on the upstream side from the orifice are detected. To detect the pressure, the pressure sensor for which semiconductor pressure sensitive elements such as a strain gauge and the like are used are widely utilized.
It has been known that, with the afore-mentioned pressure sensor to detect the fluid pressure P1, the output values change depending on the environmental conditions surrounding the sensor, for example, such as gas temperature and the like. That is, a pressure sensor placed in the same fluid pressure might have a different output value due to the changes in fluid temperature.
For example, with the afore-mentioned strain gauge type pressure sensor, pressure is converted to voltage, and the relation that the pressure on the horizontal axis corresponds with the output voltage on the vertical axis on the graph is established. And, the output characteristics that the output voltage reaches zero when the absolute pressure is zero, and the output voltage increases linearly along with the increase of the absolute pressure are desired.
However, it has been known that, with actual pressure sensors in practice, the sensor output changes even under the same gas pressure when gas temperature changes as described above, and that characteristics of pressure to output have no direct relation to each other in a strict sense.
Specifically, when the pressure applied to the pressure sensor is zero, the sensor output is called a zero point output, while when the zero point changes with temperature changes, it is called a temperature drift of the zero point output, and temperature changes of the sensor output at the time of applying pressure is called a temperature drift of the span output. Adjustments on both the temperature drift of the zero point output and the temperature drift of the span output are needed to obtain an accurate sensor output.
Let's assume, for example, that the zero point voltage is 0(V) without the temperature drift of the zero point output of the pressure sensor, and that the output voltage of the pressure sensor is 20 mV when the absolute pressure of 1.0(×102 kPaA) or the gas pressure of 1 at m is applied to the pressure sensor. When the gas temperature changes under this state, it is anticipated that the output voltage changes from 20 mV. As described above, the change is what is called the temperature drift of the span output. In fact, because of the temperature drift of the zero point output, what changed with the zero point voltage (the zero point output drift) are added to the temperature drift of the span output with any given pressure.
As explained above, with the pressure type flow rate control device and the like, while measuring the upstream side pressure P1 and/or the downstream side pressure P2, a flow rate is controlled when passing through an orifice, there are included errors with the pressure P1, P2 when the output voltage is directly converted to pressure due to the reason that the temperature change characteristics which are a temperature drift of the zero point output and the temperature drift of the span output are included in the output voltage of the pressure sensor.
For this reason, inventors of the present invention have developed system technologies which allow more accurate fluid pressure control, pressure control and flow rate control by automatically correcting the temperature drift of the zero point output and/or the temperature drift of the span output of the pressure sensor caused by the afore-mentioned temperature changes with the control circuits or control software, and made them public in TOKU-GAN No. 2001-399910.
Techniques pertaining to the afore-mentioned TOKU-GAN No. 2001-399910 make it possible to almost completely eliminate control errors on the pressure, flow rate or the like arising from such a temperature drift of the pressure sensor by employing a comparatively simply constituted device, and thereby achieve excellent, practical effects.
However, it has been recently learned that there exist not only the output voltage changes caused by the afore-mentioned fluid temperature, but also the output voltage changes over time with a pressure sensor, particularly with the pressure sensor which employs a semiconductor pressure sensitive element.
The afore-mentioned changes of the output voltage of the pressure sensor over time have become more noticeable when it is used in a state of low pressure (for example, in a vacuum of 10−4˜10−6 Torr to approx. 100 Torr) on the secondary side from the orifice F. Therefore, its influence has not been overlooked on the pressure type flow rate control device that is used for the device to supply various gases to the process chamber in semiconductor manufacturing facilities.
On the other hand, to eliminate effects due to the afore-mentioned output changes of the pressure sensor over time, it may be possible to formulate a measure wherein characteristics of pressure to output of the pressure sensor are varied for a prescribed volume by installing an additional control circuit or control software. However, this measure creates a problem because the additional installation of the device to correct these output changes over time (hereinafter called a “time-varying output drift of the pressure sensor”) invites a rise in manufacturing costs of a pressure control device or a flow rate control device.
Patent Literature 1: U.S. Pat. No. 5,146,941
Patent Literature 2: TOKU-KAI-HEI No. 8-338546 Public Bulletin
Patent Literature 3: TOKU-KAI-HEI No. 10-82707 Public Bulletin